Investigation of Magnesium Hydroxide as a Halogen-Free Fire-Retardant Filler for Advanced Polymer-Based Composite Solutions: A Review

Magnesium hydroxide is attracting growing interest as a versatile, halogen free flame retardant, and this review surveys its production routes, structure–property relationships and use in polymer systems from commodity polyolefins to advanced bio based materials. Industrial Mg(OH)₂ is still predominantly obtained from mining or hydration of MgO, but increasing attention is being devoted to recovery from seawater and saltwork brines, where precipitation from Mg²⁺ rich streams followed by controlled rehydration or direct precipitation yields fine, high purity powders suitable for flame retardant use and simultaneously valorizes saline wastes. In parallel, hydrothermal synthesis has been extensively explored to tailor particle size and morphology by adjusting precursor, solvent, temperature and time, enabling high surface area Mg(OH)₂ or MgO with narrow size distributions that are attractive for high performance composites also evaluated via ball milling crushing and refining. More recently, process intensification strategies such as microwaves and ultrasounds have been proposed to shorten reaction times, lower temperatures and better control nucleation and growth, opening paths toward energy efficient production of structured Mg(OH)₂ from both conventional and brine derived precursors. The second part of the review analyzes how the intrinsic endothermic decomposition and basic character of Mg(OH)₂ can be utilized across a broad range of polymer matrices and how surface functionalization strategies extend its applicability. In addition to “as received” powders, stearic acid and other fatty acids, metal soaps and various organic coupling agents are widely used to render the surface more hydrophobic, enhance dispersion and interfacial adhesion, and in some cases introduce additional char forming or barrier functionality. On the application side, the review compiles and compares fire and mechanical data for Mg(OH)₂ containing, polyolefins (HDPE, LLDPE, PP and EVA) used in cables and building products expandable polymers and foams, bio polymers such as PLA and PBS and elastomers with emphasis on the balance between loading level, processability, flame performance and mechanical integrity. By integrating advances in sustainable feedstocks, controlled synthesis and surface engineering with the rapidly expanding application space, this review aims to provide a comprehensive framework for designing next generation Mg(OH)₂ based flame retardant systems for both conventional and emerging polymer technologies.